Fast response temperature measurement within a gas turbine

ABSTRACT

A fast response temperature probe which may be used for a method for measuring an instantaneous temperature of a periodically changing fluid flow within a gas turbine is proposed. The temperature probe includes a substrate and a resistive element arranged at a surface of the substrate. Therein, at least at a surface of the substrate contacting the resistive element, the substrate comprises an insulating material having a thermal product of less than 1.5 kJ/(m 2 K sqrt(s)). The substrate or at least its surface is made from polyamide-imide such as for example fibre-reinforced Torlon© 5030. The temperature probe may allow measurements of instantaneous local temperatures of very fast fluctuations of more than 50 kHz at high spatial resolutions of, e.g., less than 0.5 mm 2 . The instantaneous temperature of a periodically changing fluid flow may be determined by correlating first and second sets of temperature measurements taking into account the periodicity of the periodically changing fluid flow.

FIELD OF THE INVENTION

The present invention relates to a fast response temperature probe andto a method for determining an instantaneous total temperature of aperiodically changing fluid flow within a gas turbine.

BACKGROUND OF THE INVENTION

A primary goal in the design of gas turbines or turbo-machines is tohave higher efficiencies and wider operating ranges. Thus, a substantialeffort is made to understand loss mechanisms and their origins invarious components of a gas turbine.

A reduction of losses in any component of a gas turbine used for exampleas an aircraft engine may lead to a higher efficiency of the entiresystem. Compressor and turbine stages are especially attractive for lossreductions since the principle of work exchange between them results inan over-proportional increase in net power output for a given efficiencyimprovement in either of them. This fact has lead to detailedinvestigations of the flow phenomena in blade and vane rows as well astheir unsteady interaction. The identification and localization of losssources in the flow field plays a major role in this process. On the onehand, it may permit blade and end wall modifications to be applied justat the right place. On the other hand, innovative geometries may bechecked against their actual performance.

In spite of improvements in the analysis of loss mechanisms based oncomputer simulations, experimental investigations are stillindispensable. Unsteady flow phenomena like an outflow from a rotor inthe absolute frame of reference or eddy shedding in the wake of a vaneput high demands on an experimental technique.

Suitable probes should be able to follow fluctuating flow variables suchas a local instantaneous total temperature or total pressure as theyexpress a change in entropy which is an indicator for loss mechanismsand, finally, the efficiency. In order to obtain precise results havinga higher time resolution than the order of magnitude of the “bladepassing frequency” and both shorter and smaller than the order ofmagnitude of a time scale and dimension of turbulences, both a very hightemporal and spatial resolution may be required for the measurements.The required sophistication of such measurement systems becomes obviousby considering the fluctuation frequencies of unsteady flow variables inactual test rigs. In order to be able to resolve typical temperaturefluctuation frequencies of 50 kHz, measurements may need to be acquiredat acquisition rates of 100 kHz or more.

SUMMARY OF THE INVENTION

There may be a need to determine a fluctuating temperature within a gasturbine with a high temporal resolution of for example more than 50 kHzand with a high spatial resolution of for example less than 1 mm².Particularly, there may be a need to determine the total temperature,also referred to as stagnation temperature, within turbine test rigswith such high temporal and spatial resolution. Particularly, there maybe a need for a fast response temperature probe being adapted foracquiring temperature data at such high temporal and spatial resolution.

Furthermore, there may be a need for a method for measuring aninstantaneous total temperature of a periodically changing fluid flowwithin a gas turbine, the method allowing for acquiring a localtemperature at such high temporal and spatial resolution.

Preferably, both the temperature probe and the temperature measuringmethod should be adapted to allow miniaturization and robustness suchthat high frequency temperature measurements in a harsh environmentwithin a gas turbine test rig may be achieved.

These needs may be met with the subject-matter as defined in theappended independent claims. Advantageous embodiments are described inthe dependent claims.

According to a first aspect of the present invention, a fast responsetemperature probe comprises a substrate and a resistive element. Theresistive element has a temperature-dependent electrical resistance andis arranged at a surface of the substrate. Therein, at least at asurface of the substrate contacting the resistive element, the substratecomprises an insulating material having a thermal product of less than1.5 kJ/(m²K√{square root over ((s))}).

In order to measure temperatures, it is well-known to measure atemperature-dependent parameter such as an electrical resistance of aresistive element. Conventionally, such resistive element may beprovided as a small wire through which an electrical current may betransferred and the electrical resistivity of which can be determined inorder to obtain an indication of the temperature of the wire. However,in order to have a sufficient mechanical strength, such wire usuallyneeds to have substantial dimensions, especially a substantial diameter.As the volume, the temperature of which is determined by the resistiveelement, is mainly determined by the dimensions of the resistiveelement, the spatial resolution of temperature measurements using suchconventional resistive element is relatively low. Furthermore, due tothe substantial dimensions of the resistive elements, the thermalcapacity of such wire may be relatively high such that the wire revealscertain inertia when following fast temperature fluctuations.Accordingly, the temporal resolution of temperature measurements usingsuch conventional resistive element is relatively low.

It is proposed herein to provide a resistive element by arranging it ata surface of a substrate. Using such substrate as a carrier, thesubstrate may provide for the necessary mechanical strength and thus,the dimensions of the resistive element may be strongly reduced.Thereby, the resistive element used for the temperature probe may bereduced in its overall dimensions thereby allowing for an increasedspatial resolution. In other words, as the resistive element may beminiaturized by arranging it on a carrier substrate, the dimensions of avolume the temperature of which is to be measured may be reduced.

However, as the resistive element is arranged on a substrate, heattransfer between the resistive element and the substrate should beminimized as far as possible in order to reduce inertia of thetemperature probe due to such heat transfer. In other words, it shouldbe possible to heat the resistive element very fast without necessarilyheating the substrate or its surface at the same rate.

Therefore, it is proposed to use a substrate which, at least at asurface of the substrate contacting the resistive element, comprises athermally insulating material having a thermal product of less than 1.5kJ/(m²K sqrt(s)) (=1.5 kJ/(m²K√{square root over ((s))})), preferablyless than 1.0 kJ/(m²K sqrt(s)), and more preferably less than 0.85kJ/(m²K sqrt(s)) with a suitable thickness. Therein, the thermal producttp may be defined as the square root of the density ρ times the thermalcapacity c times the thermal conductivity k (tp=sqrt(ρ*c*k) which is anindicator of the isolation properties of the substrate material.

When the proposed fast response temperature probe is brought in contactwith a fluid such as a gas or a liquid showing fast temperaturefluctuations, the temperature of the miniaturized resistive element mayquickly follow the temperature fluctuations as, due to the low thermalproduct of the material of the substrate contacting the resistiveelement, there is only minimal heat transfer from the resistive elementto the substrate surface. Accordingly, such temperature probe may allowmeasuring temperature profiles with very high temporal resolution of forexample more than 50 kHz.

Furthermore, due to the low thermal product of the insulating materialadjacent to the resistive element, the resistive element may be easilyheated to substantial temperatures upon passing a constant electricalheating current through the resistive element. Accordingly, by applyinga low electrical current to the resistive element, it may be easilyheated to an elevated wall temperature. This may be advantageously usede.g. in a method for measuring an instantaneous total temperature of afluid further described below with respect to a second aspect of theinvention.

Advantageously, the insulating material of the substrate contacting theresistive element comprises polyimide or polyamide-imide.

Polyamide-imides, also referred to as PAI, are polymers comprising bothamide groups and imide groups. Polyamide-imides with aromatic componentsin the polymer chain are highly thermally stable. The temperatureresistance may be at up to 450° C. Furthermore, they provide for a goodchemical and mechanical resistance. The thermal product ofpolyamide-imides is typically between 0.5 and 0.85 kJ/(m²K sqrt(s)),more often between 0.6 and 0.8 kJ/(m²K sqrt(s)).

Advantageously, the substrate comprises a fibre-reinforced material. Thefibre-reinforcement may provide for an improved rigidity or mechanicalstrength of the substrate. Particularly in a case, where the substrateis provided as a long, thin rod, the stiffness of such rod may beincreased by reinforcing the substrate material with fibres such asglass fibres.

Advantageously, the substrate comprises Torlon©. Torlon© is a trade nameof a specific polyamide-imide provided by Solvay Advanced Polymers.Torlon© may provide for exceptional long-term strength and stiffness upto 275° C., outstanding wear resistance, superior toughness fromcryogenic temperatures up to 275° C., resistance to strong acids andmost organics, inherent flame resistance and low CLTE. Torlon© 4203L hasa thermal product of approximately 0.66 kJ/(m²K sqrt(s)).

Alternatively, the substrate comprises Polyimide like PI2525 from HDMicrosystems having a thermal product of 0.63 kJ/(m²K sqrt(s)) and beingusually used for electrical devices and wafer planarization.

Advantageously, Torlon© 5030 may be used for the substrate. Torlon© 5030is a polyamide-imide reinforced with 30% glass fibre. It provides forenhanced mechanical stiffness. For example, a thin rod made of Torlon©5030 may be used as a substrate showing both, high mechanical stiffnessand low thermal product.

It has been observed that a substrate made from a fibre-reinforcedmaterial such as Torlon© 5030 may have a rather rough surface unless itis specifically treated or finished. Surface roughnesses in the order ofup to 5 μm have been observed. Due to such surface roughness, problemsmay occur when providing a resistive element with very small dimensionsonto such rough surface. For example, in case of a resistive elementcomprising narrow conducting stripes having a width in the order of 10μm, interruptions or shortcuts may occur. Furthermore, as the resistiveelement may have a thickness of less than 500 nm, a surface roughness ofup to 5 μm may result in further problems such as e.g. a not completesurface coverage in the coating process due to shadowing effects.Therefore, it may be advantageous to planarize or polish the surface ofa fibre reinforced substrate before depositing the resistive elementthereon.

In another advantageous approach, the insulating material is provided asa coating onto a substrate base. For example, an insulating material oflow thermal product such as Torlon 4203© (Solvays trade name for Torlon4203© paint is Torlon AI-10) or PI2525 may be coated with a suitablethickness onto an arbitrary substrate base wherein the substrate baseitself does not necessarily need to have a very low thermal product.When the coating has a sufficient thickness of for example between 1 μmand 100 μm, preferably between 5 μm and 60 μm, no significant heattransfer between the substrate base and the resistive element, therebypassing through the coating, should occur and accordingly the resistiveelement should be able to follow temperature fluctuations with lowthermal inertia.

In order to provide for good mechanical stiffness, the substrate basemay be provided with a fibre reinforced material. For example, thesubstrate base may be provided with Torlon© 5030. A possibly roughsurface of such fibre reinforced substrate material may be equalized orplanarized by the coating which itself is not fibre-reinforced.

Advantageously, the insulating material at the surface of the substratecomprises a coefficient of linear thermal expansion of less than 50×10⁶1/° K, preferably less than 20×10⁶ 1/° K. Besides their low thermalproducts, there usually exists a gross discrepancy in the coefficient oflinear thermal expansion β of metals usually used for the resistiveelement and conventional polymers usually used for a substrate. Herein,the coefficient of linear thermal expansion β is defined by:β(T)=1/L*dL/dT. Herein, L is the length of the object. The coefficient βis either given at a certain temperature T or as an average value over atemperature range.

Since the resistive element of the proposed temperature probe is indirect thermal contact with a surface of the substrate and isperiodically heated up and cooled down during operation, a difference inthermal expansion of the resistive element and the substrate may inducethermal stresses. Accordingly, it may be advantageous to use materialsfor the resistive element and materials for the substrate having atleast similar coefficients of linear thermal expansion β. For example,possible materials for the resistive element may be nickel (Ni) having acoefficient of linear thermal expansion β of 13.75×10⁻⁶ 1/° K in atemperature range of 0 to 200° C. or platinum (Pt) having a β of9.15×10⁻⁶ 1/° K in a temperature range of 0 to 200° C. Advantageousinsulating materials for the substrate may be Torlon© 5030 with a β offor example 16.2×10⁻⁶ 1/° K or Torlon© 4203 with a β of for example30.6×10⁻⁶ 1/° K. An alternative material is PEEK (PolyEtherEtherKetone)typically having a β of 20-50×10⁻⁶ 1/° K, polyimide such as PI2525typically having a β of 20-40×10⁻⁶ 1/° K or LCP (Liquid ChrystalPolymer) with a β of typically 20×10⁻⁶ 1/° K.

Advantageously, the resistive element comprises a thin film gaugeincluding a meander-shaped metal thin film. In other words, theresistive element may be made with a thin conducting stripe beingarranged in a meander-pattern or serpentine-pattern. The conductingmetal thin film may have a thickness of between 10 and 1.000 nm,preferably between 100 and 500 nm, and a width of less than 50 μm,preferably between 2 μm and 20 μm. The meander-pattern orserpentine-pattern may be arranged such that a maximum length of metalstripes is provided on a minimum surface area. For example, the patternmay have a rectangular, square or circular shape. For example, thepattern may extend over a surface area of less than 1 mm², preferablyless than 0.2 mm².

The metal thin film may comprise nickel and/or platinum or consist ofnickel or platinum which, due to their high resistivity and hightemperature coefficient of resistance, allow for a high temperaturesensitivity of the resulting resistive element.

Advantageously, the substrate is rod-shaped having a surface-shell withlow aerodynamic resistance to a fluid flow perpendicular to therod-shaped substrate. In other words, the substrate may have an elongateshape a cross-section of which is formed such that a fluid may flowaround the substrate in a direction perpendicular to the longitudinalaxis of the substrate with low or minimum aerodynamic resistance. Asfurther described below, such geometric shape may be advantageous whenusing the temperature probe for measuring a fluid flow of high velocityfor example within a gas turbine. Therein, the advantageous aerodynamicshape of the substrate may prevent excessive bending forces onto thesubstrate during the measuring process. For example, the substrate maybe provided with a cylindrical or semi-cylindrical shape.

Advantageously, the temperature probe comprises exactly one resistiveelement. In other words, the temperature probe does not comprise two ormore resistive elements. As will be more apparent from the descriptionof the method for measuring a temperature in accordance with a secondaspect of the present invention as described below, it may be sufficientto provide the temperature probe with only one resistive element whichmay then be operated under different operating conditions. Using onlyone resistive element may provide for a reduced temperature measuringarea compared to approaches where the temperature is measured with twoor more resistive elements provided on a same temperature probe.

According to a second aspect of the present invention, a method formeasuring an instantaneous total temperature of a periodically changingfluid flow within a gas turbine is proposed. The method comprises: (a)positioning a fast response temperature probe at a position within thefluid flow; (b) acquiring a first set of temperature measurements T_(w1)with the fast response temperature probe by applying a constantmeasurement current; (c) setting the fast response temperature probe toa different temperature; (d) acquiring a second set of temperaturemeasurements T_(w2) with the fast response temperature probe; and (e)determining the instantaneous total temperature by correlating the firstand second sets of temperature measurements T_(w1) T_(w2) taking intoaccount the periodicity of the periodically changing fluid flow in thegas turbine.

The method steps (a) to (e) may be performed in the indicated order,i.e. first, a first set of temperature measurements T_(w1) is acquiredand possibly stored within a memory, then the temperature of thetemperature probe, particularly of its temperature detection surface, ischanged by changing the current, and then a second set of temperaturemeasurements T_(w2) is acquired and possibly stored in a memory.Therein, T_(w1) and T_(w2) may be the wall temperatures of thetemperature probe where one wall temperature T_(w1) may substantiallycorrespond to an averaged temperature of fluid contacting the probe'swall and the other wall temperature T_(w2) is higher.

In other words, during measuring the first set of temperatures T_(w1),the temperature probe may not be heated, i.e. no significant current ispassed through the resistive element of the temperature probe, and theinstantaneous temperature of the resistive element substantiallycorresponds to the instantaneous temperature of the fluid being incontact therewith. During measuring the second set of temperaturesT_(w2), the temperature probe may be heated, i.e. a significant constantcurrent is passed through the resistive element of the temperatureprobe, thereby heating the probe to an elevated temperature. At thiselevated temperature, temperature fluctuations in the resistive elementdue to temperature fluctuations in the fluid contacting the resistiveelement may be measured. The wall heat fluxes {dot over (q)}₁ and {dotover (q)}₂ will be different during measurement of the first set oftemperatures T_(w1) and the second set of temperatures T_(w2),respectively due to the different temperatures of the probe wall.

Finally the acquired temperature measurement sets may be used todetermine instantaneous total temperatures at each point in time in atime-dependent temperature profile. Therein, it may be advantageouslyused that the fluid flow changes its temperature periodically with eachrotor revolution and that the periodicity of such change may bedetermined such that the instantaneous temperature may be determined foreach phase of the period by using the temperature measurements of therespective phase point in time from the first set of temperaturemeasurements T_(w1) and the second set of temperature measurementsT_(w2). For determining the periodicity of the periodically changingfluid flow, knowledge on the continuously changing rotation position ofa rotor within the gas turbine and of the position of the bladesarranged at its circumference may be used.

Having determined the wall temperatures T_(w1) and T_(w2) as well ascalculated the wall heat fluxes {dot over (q)}₁ and {dot over (q)}₂, thetotal temperature T_(t), also referred to as stagnation temperature, maybe determined at the location of the temperature detection surface ofthe temperature probe.

The principle of this gas temperature measurement is based on twotemperature measurements with different probe temperatures assuming theheat transfer coefficient h to be identical for both measurements (whichmay be assumed as a result of the periodicity of the fluid flow andassuming a fixed measurement position):

{dot over (q)} ₁ =h(T _(t) −T _(w1))

{dot over (q)} ₂ =h(T _(t) −T _(w2))

The evaluation of the heat fluxes at the wall {dot over (q)}₁ and {dotover (q)}₂ from surface temperature resolution may be done as follows: Athin metallic conductor stripe deposited on an isolating substrate mayallow monitoring the surface temperature by its resistance change. Thisboundary condition being known as well as the substrate temperature at agiven depth, one can solve the unsteady heat conduction equation in thesubstrate:

$\frac{\partial T}{\partial t} = {a\frac{\partial^{2}T}{\partial x^{2}}}$

Herein, a 1D case without source terms and constant properties isassumed. The solution can be performed by analogue or numericaltechniques like the Crank-Nicolson schema.

The implicite Crank-Nicholson discretization of the 1D heat conductionequation has the form:

$\frac{T_{i}^{n + 1} - T_{i}^{n}}{\Delta \; t} = {a\frac{T_{i + 1}^{n + {1/2}} - {2\; T_{i}^{n + {1/2}}} + T_{i - 1}^{n + {1/2}}}{\Delta \; x^{2}}}$

The wall heat flux at the n_th time step is computed from the first twogrid points i=0 and i=1

${\overset{.}{q}}_{w} = {{- k}{\frac{T_{1}^{n} - T_{0}^{n}}{\Delta \; x}.}}$

Further information on such calculation may be derived from Minkowycz,W. J.: “Handbook of numerical heat transfer”, 2. Ed, Hoboken, N.J.:Wiley, 2006, ISBN 0-471-34878-3 978-0-471-34878-8, pp. 73 ff.

The heat fluxes of the wall being known as well as the surfacetemperature, the above equations may be rewritten as:

$h = \frac{{\overset{.}{q}}_{1} - {\overset{.}{q}}_{2}}{T_{w\; 2} - T_{w\; 1}}$$T_{t} = {T_{w\; 1} + {{\overset{.}{q}}_{1} \cdot \frac{T_{w\; 2} - T_{w\; 1}}{{\overset{.}{q}}_{1} - {\overset{.}{q}}_{2}}}}$

Accordingly, as a result, these two measurements allow the determinationof the heat transfer coefficient h and the gas total temperature T_(t)at a specific time t.

Furthermore, a transition between the two wall temperatures may also beacquired and stored in order to extract further information e.g. on aheat transfer coefficient by numerical postprocessing.

Advantageously, the first and second sets of temperature measurementsT_(w1), T_(w2) are acquired at a same position within the fluid flow. Inother words, the temperature probe may remain static during the entiremeasuring process and, finally, a value of the gas total temperature atthe position of the temperature probe's temperature detection surfacemay be provided. Accordingly, for acquiring both, the first and thesecond sets of temperature measurements T_(w1), T_(w2), providing only asingle temperature probe is sufficient as the sets of temperaturemeasurements may be acquired one after the other. The only onetemperature probe may remain static at the same position such that thespatial resolution of the temperature measurement is mainly determinedby the temperature detection surface of the temperature measurementprobe.

Advantageously, the fast response temperature probe is set to adifferent temperature by inducing an electrical heating current throughthe temperature probe. In other words, the temperature probe maycomprise a resistive element for acquiring the sets of temperaturemeasurements T_(w1), T_(w2). On the one hand, the resistive element maybe used to determine an instantaneous temperature by determining theinstantaneous temperature-dependent resistance of the resistive elementdue to changes in the fluid. On the other hand, a constant bias currentthrough the resistive element may be used to heat the resistive elementduring temperature measurement acquisition by generating Joule's heat.Accordingly, by inducing such electrical heating current, the walltemperature of the temperature probe may be set to a differenttemperature compared to a case without such electrical heating current.Alternatively, different heating currents may be passed through theresistive element during acquiring the first and second measurement set,respectively, thereby heating the temperature probe's temperaturedetection surface to different wall temperatures.

Advantageously, the first and second sets of temperature measurementsT_(w1), T_(w2) are acquired at an acquisition rate of more than 20 kHz,preferably more than 80 kHz and more preferably more than 160 kHz. Forexample, when using the fast response temperature probe as describedfurther above with respect to the first aspect of the invention, thefirst and second sets of temperature measurements T_(w1), T_(w2) may beacquired with very high acquisition rates of between 80 kHz andapproximately 500 kHz. With such high acquisition rates, temperaturefluctuations within the fluid flow may be determined at very hightemporal resolution which may allow to better analyze physical processesfor example occurring in a fluid flow through a gas turbine.

It has to be noted that aspects and embodiments as well as features andadvantages of the present invention are described herein with referenceto different subject-matters. In particular, some embodiments have beendescribed with reference to the proposed fast response temperature probewhereas other embodiments have been described with reference to themethod of measuring an instantaneous temperature in a fluid flow.However, a person skilled in the art will gather from the above and thefollowing description that, unless other notified, in addition to anycombination of features belonging to one type of subject-matter also anycombination between features relating to different subject-matters isconsidered to be disclosed with this application.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the present invention will be furtherdescribed with reference to specific embodiments as shown in theaccompanying figures but to which the invention shall not be limited.

FIG. 1 shows a conventional gas turbine.

FIG. 2 illustrates a spatial temperature distribution within a turbineexhaust region of a gas turbine.

FIG. 3 illustrates time dependence of a temperature at a specificmeasuring site within a turbine exhaust region.

FIG. 4 shows a fast response temperature probe according to anembodiment of the present invention.

FIG. 5 shows a meander-pattern for a resistive element in a temperatureprobe according to an embodiment of the present invention.

All figures are only schematically and not to scale. Same referencesigns refer to same or similar elements throughout the figures.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a conventional gas turbine 100 as can be used for examplefor an aircraft. The gas turbine comprises a fan 102 rotating within aninlet case 104 in front of a low pressure compressor 106. In a centralregion of the turbine 100, a high pressure compressor 108 is arranged infront of a combustion chamber 110. Inlet air entering the turbine 100through the fan is compressed in the low and high pressure compressorbefore being fed to the combustion chamber. In the combustion chamber,fuel is added to the inlet air and combusted. The expanding combustiongases then enter a high pressure turbine 112 including rotating turbineblades 114 and finally pass through a low pressure turbine 116. Theblades 114 of the high pressure turbine 112 and the low pressure turbine116 are arranged within a turbine exhaust case 118.

FIG. 2 shows an enlarged visualization of a spatial temperaturedistribution in a region between a set of turbine blades 114 of the lowpressure turbine 116 enclosed within the turbine exhaust case 118. Ascan be seen from FIG. 2, there are significant temperatureinhomogeneities within the gas flow exiting the turbine exhaust. As thisgas flow exits the turbine exhaust at velocities of approximately Mach1,the temperature fluctuations within the gas flow pass very quickly alonga specific position indicated by the tip 2 of a fast responsetemperature probe 1 introduced into the gas flow through a small openingwithin the turbine exhaust case 118.

FIG. 3 shows time-dependent temperature fluctuations at a specificlocation of a probe 1 arranged within a gas flow in a gas turbineadjacent to blades 114.

For experiments serving for analyzing gas flow properties within a gasturbine, test rigs may be used in which a gas flow is forced through agas turbine however without combusting fuel within the combustionchamber. Accordingly, there are no highly elevated temperatures withinthe gas flow. Typical gas temperatures may be in a range of 40 to 60° C.up to 200° C. However, the temperature distribution within the gas flowmay be similar to the actual operating conditions at high temperaturesand may provide valuable information on the gas flow characteristics.

FIG. 4 shows a fast response temperature probe 1 according to anembodiment of the present invention. In the figure, a tip 2 of thetemperature probe 1 is separately shown at an enlarged scale. Thestructure of the resistive element 3 provided at the tip 2 of thetemperature probe 1 is schematically shown in FIG. 5.

A substrate 5 of the temperature probe 1 comprises a cylindrical rodmade from a glass fibre-reinforced polyamide-imide Torlon© 5030 having adiameter of 1 mm. Due to the glass fibre-reinforcement, even a rod withsuch small diameter has a sufficient rigidity for being positionedwithin a fast flowing fluid within the turbine exhaust case. A surfaceof the rod substrate has been polished or coated with another Torlon©coating in order to planarize the surface. On the cylindrical surface ofthis substrate rod, a nickel thin film having a thickness of 100 nm isdeposited using for example PLD (Pulsed Laser Deposition) or sputteringdeposition.

In the nickel thin film, conducting stripes are created in ameander-like fashion using a laser on a surface area of 350 μm by 400μm. As shown in FIG. 5, the conducting stripes of the meander-patternhave a width of approximately 10 μm and a length of approximately 400 μmand are arranged side by side along an overall width of approximately350 μm. The substrate rod is coated with a gold layer of 1 μm thicknessin order to provide contacts to be contacted by external measuringdevices.

With such fast response temperature probe 1, the local temperature of afluid directly adjacent to the resistive element 3 may be measured at avery fast rate. With the nickel thin film providing the resistiveelement 3, temperature measurements can be achieved at a frequency of upto 88 kHz. It has been calculated that with a metal thin film layer madeof platinum, frequencies of even up to 470 kHz may be attained. Thespatial resolution is in the order of magnitude of the surface of theresistive element 3, i.e. 350 μm by 400 μm=0.14 mm².

Furthermore, as the thermal expansion of the Torlon© 5030 rod substrateis substantially the same as the thermal expansion of the metal used forthe resistive element 3, failures due to cracks occurring as a result ofdiffering thermal expansion coefficients of adjacent materials may beeffectively prevented. Furthermore, errors resulting from changes of anelectrical resistance due to mechanical strains or compressions may beminimized.

Using such fast response temperature probe 1 having a single resistiveelement 3 of very small detection surface, the method of measuring aninstantaneous temperature of a periodically changing fluid flow withinthe gas turbine may be advantageously performed. First, the temperatureprobe 1 may be inserted into the fluid flow within the gas turbinethrough a small hole within the turbine exhaust case. The temperatureprobe is fixedly positioned and a first set of temperature measurementsT_(w1) of the passing gas flow is acquired during a first time span.During this first time span, a first constant electrical current ispassed through the resistive element 3. This first electrical currentmay be chosen as low as to only negligibly heat the resistive element 3thereby. Due to resistance changes a resistive voltage change may berecorded with a suitable measurement device. Then, the electricalcurrent through the resistive element 3 is changed to another constantvalue thereby heating the resistive element 3 to a differenttemperature. As soon as the temperature within the resistive element 3has stabilized, a second set of temperature measurements T_(w2) isacquired during a second time span. During the second time span, theelectrical current through the resistive element again remains constant,i.e. the temperature measurement is made at constant current (CCAmeasurement). The first and second sets of temperature measurementsT_(w1), T_(w2) acquired at different constant electrical currentsresulting in different wall temperatures and different heat fluxes maybe used to finally calculate the total temperature of the gas at thelocation of the resistive element 3. In calculating the totaltemperature, the periodic characteristics of the turbine gas flow andthe assumption of the same heat transfer coefficient for bothtemperatures of the resistive element may be used. By additionallystoring the actual position within a revolution of the turbine and ofthe respective blade positions, the measured values may be correctlyassigned and averaged. The assignment may be done based on a knownrotational frequency or an angle position of a rotation axis.

It should be noted that the term “comprising” does not exclude otherelements or steps and that the indefinite article “a” or “an” does notexclude the plural. Also elements described in association withdifferent embodiments may be combined. It should also be noted thatreference signs in the claims shall not be construed as limiting thescope of the claims.

LIST OF REFERENCE SIGNS

-   1 Fast response temperature probe-   2 probe tip-   3 Resistive element-   5 Substrate-   100 Gas turbine-   102 Fan-   104 Inlet case-   106 Low pressure compressor-   108 High pressure compressor-   110 Combustion chamber-   112 High pressure turbine-   114 Turbine blades-   116 Low pressure turbine-   118 Turbine exhaust case

1. A fast response temperature probe comprising: a substrate; aresistive element; wherein the resistive element has atemperature-dependent electrical resistance and is arranged at a surfaceof the substrate; wherein, at least at a surface of the substratecontacting the resistive element, the substrate comprises an insulatingmaterial having a thermal product of less than 1.5 kJ/(m² K sqrt(s)). 2.The probe according to claim 1, wherein the insulating materialcomprises at least one of polyimide and polyamidimide.
 3. The probeaccording to claim 1, wherein the substrate comprises a fiber-reinforcedmaterial.
 4. The probe according to claim 1, wherein the substratecomprises Torlon©
 5030. 5. The probe according to claim 1, wherein theinsulating material is provided as a coating onto a substrate base. 6.The probe according to claim 1, wherein the insulating materialcomprises a coefficient of linear thermal expansion of less than 50*10⁻⁶1/° C.
 7. The probe according to claim 1, wherein the resistive elementcomprises a thin film gauge including a meander-shaped metal thin film.8. The probe according to claim 7, wherein the metal thin film comprisesat least one of nickel and platinum.
 9. The probe according to claim 1,wherein the substrate is rod-shaped having a surface shell with lowaerodynamic resistance to a fluid flow perpendicular to the rod-shapedsubstrate.
 10. The probe according to claim 1, wherein the probecomprises exactly one resistive element.
 11. A method for measuring aninstantaneous total temperature of a periodically changing fluid flowwithin a gas turbine, the method comprising: positioning a fast responsetemperature probe at a position within the fluid flow; acquiring a firstset of temperature measurements T_(w1) with the fast responsetemperature probe; setting the fast response temperature probe to adifferent wall temperature; acquiring a second set of temperaturemeasurements T_(w2) with the fast response temperature probe;determining the instantaneous total temperature by correlating the firstand the second sets of temperature measurements T_(w1), T_(w2) takinginto account the periodicity of the periodically changing fluid flow.12. The method of claim 11, wherein the first and second sets oftemperature measurements T_(w1), T_(w2) are acquired at a same position.13. The method of claim 11, wherein the fast response temperature probeis set to a different temperature by inducing an constant electricalheating current through the temperature probe.
 14. The method of claim11, wherein the first and second sets of temperature measurementsT_(w1), T_(w2) are acquired at an acquisition rate of more than 100 kHz.15. The method of claim 11, further comprising: using a fast responsetemperature probe for acquiring the first and second sets of temperaturemeasurements T_(w1), T_(w2).